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Geology of the Upper Jurassic to Lower Cretaceous
geothermalaquifers in the West Netherlands Basin - an overview
Citation for published version:Willems, CJL, Vondrak, A,
Mijnlieff, HF, Donselaar, ME & van Kempen, BMM 2020, 'Geology
of the UpperJurassic to Lower Cretaceous geothermal aquifers in the
West Netherlands Basin - an overview',Netherlands Journal of
Geosciences, vol. 99, 1. https://doi.org/10.1017/njg.2020.1
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Netherlands Journal ofGeosciences
www.cambridge.org/njg
Original Article
Cite this article: Willems CJL, Vondrak A,Mijnlieff HF,
Donselaar ME, andvan Kempen BMM. Geology of the UpperJurassic to
Lower Cretaceous geothermalaquifers in the West Netherlands Basin –
anoverview. Netherlands Journal of Geosciences,Volume 99, e1.
https://doi.org/10.1017/njg.2020.1
Received: 1 July 2019Revised: 21 November 2019Accepted: 20
January 2020
Keywords:hot sedimentary aquifer, direct-use, lowenthalpy, North
Sea basin, reservoir geology
Author for correspondence:Cees J.L. Willems,Email:
[email protected]
© The Author(s) 2020. This is an Open Accessarticle, distributed
under the terms of theCreative Commons Attribution licence
(http://creativecommons.org/licenses/by/4.0/), whichpermits
unrestricted re-use, distribution, andreproduction in any medium,
provided theoriginal work is properly cited.
Geology of the Upper Jurassic to LowerCretaceous geothermal
aquifers in the WestNetherlands Basin – an overview
Cees J.L. Willems1 , Andrea Vondrak2, Harmen F. Mijnlieff3 ,
Marinus E. Donselaar4,5 and Bart M.M. van Kempen3
1School of Engineering, University of Glasgow, Glasgow, UK;
2TAQA Energy B.V., Alkmaar, the Netherlands;3TNO – Geological
Survey of the Netherlands, P.O. Box 80015, 3508 TA Utrecht, the
Netherlands; 4Department ofGeoscience and Engineering, Delft
University of Technology, Delft, the Netherlands and 5Division of
Geology,Department of Earth and Environmental Sciences, KU Leuven,
Leuven, Belgium
Abstract
In the past 10 years the mature hydrocarbon province the West
Netherlands Basin has hostedrapidly expanding geothermal
development. Upper Jurassic to Lower Cretaceous stratafrom which
gas and oil had been produced since the 1950s became targets for
geothermalexploitation. The extensive publicly available subsurface
data including seismic surveys, severalcores and logs from hundreds
of hydrocarbon wells, combined with understanding of the geol-ogy
after decades of hydrocarbon exploitation, facilitated the offtake
of geothermal exploitation.Whilst the first geothermal projects
proved the suitability of the permeable Upper Jurassic toLower
Cretaceous sandstones for geothermal heat production, they also
made clear that muchdetail of the aquifer geology is not yet fully
understood. The aquifer architecture varies signifi-cantly across
the basin because of the syn-tectonic sedimentation. The graben
fault blocks thatcontain the geothermal targets experienced a
different tectonic history compared to the horstand pop-up
structures that host the hydrocarbon fields from which most
subsurface data arederived. Accurate prediction of the continuity
and thickness of aquifers is a prerequisite forefficient geothermal
well deployment that aims at increasing heat recovery while
avoidingthe risk of early cold-water breakthrough. The potential
recoverable heat and the current chal-lenges to enhance further
expansion of heat exploitation from this basin are evident. This
paperpresents an overview of the current understanding and
uncertainties of the aquifer geology ofthe Upper Jurassic to Lower
Cretaceous strata and discusses new sequence-stratigraphicupdates
of the regional sedimentary aquifer architecture.
Introduction
TheWest Netherlands Basin (WNB) is a prolific hydrocarbon
province with some 80 oil and gasfields. Interest in its
exploration was accidentally initiated at the world exhibition of
1938 inThe Hague, when the Bataafse Petroleum Maatschappij (BPM)
drilled a demonstration wellon the De Mient exhibition site. It
struck oil at 460 m (Knaap & Coenen, 1987). During andshortly
after World War II, exploration began. Seismic and gravimetric
surveys were acquired,and wells were drilled, resulting in the
discovery of the first oilfield, the Rijswijk Field, in 1953.The
first gas discovery in the WNB, the Botlek Field, followed in 1984.
The last gas discoverydates from 2016 in the P11 block. The main
target horizons in this basin were Upper Jurassic toLower
Cretaceous and Triassic strata. Oil and gas exploration and
production resulted in awealth of seismic and well data. Almost the
entire WNB is covered by 3D-seismic surveys.They are of different
vintage, and the standard public domain versions are of reasonable
quality.A giant recent leap in data availability in the public
domain (www.nlog.nl) is the release ofreprocessed versions of the
3D-seismic and of raw well data of all drilled wells. In addition,a
wealth of published literature is available (see e.g. Rondeel et
al., 1996). De Jager et al.(1996), Den Hartog Jager (1996) and
Racero-Baena & Drake (1996) provide comprehensiveoverviews of
the Upper Jurassic to Lower Cretaceous sedimentary deposits,
hydrocarbon gen-eration and structural style of the WNB.
At present, hydrocarbon exploration is almost halted and most of
the fields in the matureWNB hydrocarbon province are abandoned or
in their tail-end production stage. Since themid-1970s the area has
entered its second life for geothermal exploration (e.g.
Lokhorst,2000). Several geothermal potential desktop studies were
conducted between then and themid-1990s. These studies already
identified Lower Cretaceous Sandstone beds as main geother-mal
targets in the WNB, culminating in the definition of the Delfland
geothermal project(Dufour, 1984). However, it took until 2005
before the first geothermal operator initiated ageothermal project
to replace the gas-powered heating system in this greenhouse
complex with
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a geothermal heating system against a backdrop of high and
volatilegas prices (Ramaekers et al., 2006). Aided by governmental
support,a successful doublet system was realised that has exploited
the sedi-mentary rocks of the Nieuwerkerk and Vlieland formations
since2007. This project initiated several follow-up projects in the
WNB.At present, 12 doublets are realised in the Upper Jurassic to
LowerCretaceous strata in the WNB. Initially, the marine sandstones
ofthe Vlieland Formation were the main target. However,
interestshifted rapidly to the Delft Sandstone Member (Delft Sst
Mbr)of the underlying Nieuwerkerk Formation because of its
highertemperature, permeability and thickness (Donselaar et
al.,2015; Willems et al., 2017c; Vondrak et al., 2018). Recentlythe
deeper Alblasserdam Member (Alblasserdam Mbr) of theNieuwerkerk
Formation has been targeted, and ongoing explora-tion aims for a
combined development of both stratigraphicintervals. In 2017 the
first geothermal system, exploiting thiscombined aquifer, was
realised.
Recent geothermal exploration renewed interest in the
UpperJurassic to Lower Cretaceous strata of the deeper graben
faultblocks of the WNB. Newly acquired data from geothermal
wellsfrom different palaeo tectono-sedimentary locations warrant
revis-iting of the tectono-sedimentological and
sequence-stratigraphicalmodels to better understand and predict the
lateral and verticaldistribution and quality variations of the
different aquifer zones.This paper presents an overview of the new
insights into theDelft Sst Mbr reservoir architecture and the
geothermal potentialof the Vlieland Sst Formation.
Structural geological setting, West Netherlands Basin
The WNB is a northwest–southeast-trending basin in the south
ofthe Netherlands extending westward into the southern DutchNorth
Sea. The basin is flanked by the London–Brabant High inthe
southwest and the Zandvoort Ridge in the northeast (Fig.
1).Extensional movement commenced in the Middle Jurassic, creat-ing
a series of parallel half-grabens (Bodenhausen & Ott, 1981;Den
Hartog Jager, 1996; Racero-Baena & Drake, 1996). The sub-siding
half-grabens were filled with terrestrial sediments sourcedfrom the
London–Brabant Massif in the south and from theRoer Valley Graben
in the southeast (Den Hartog Jager, 1996;Herngreen & Wong,
2007). The syn-tectonic deposition of thesesediments is reflected
by a major unconformity at the base ofthe Upper Jurassic to Lower
Cretaceous strata and by the widelydiffering thickness of these
strata in the adjacent fault blocks(Fig. 2). In addition, local
intra-Nieuwerkerk unconformitiesare evidence of various local
tectonic events (e.g. Devault &Jeremiah, 2002). Around
Hauterivian times, the basin entered apost-rift sag phase, while
relative sea level was rising. The UpperCretaceous sediments are
therefore more homogeneous in thick-ness, wedging towards the basin
margins (Fig. 2; Den HartogJager, 1996; Vondrak et al., 2018). The
palaeo-coastline trans-gressed from the northwest boundary of the
WNB in theRyazanian to the northwest boundary of the Roer Valley
GrabeninHauterivian times, which is reflected by increasingly
marine sed-imentation covering the terrestrial syn-tectonic strata
(Den HartogJager, 1996; Herngreen and Wong, 2007; Jeremiah et al.,
2010;Vejbbæk et al., 2010).
A new tectonic phase in the Palaeocene significantly alteredthe
structural setting of the basin as part of the Late
CretaceousLaramide compressional phase (Van Wijhe, 1987; Deckers
&van der Voet, 2018). During this Alpine inversion phase,
manyof the Jurassic normal faults were reactivated as reverse
faults.
Due to inversion and uplift of horst and pop-up structures,
theCretaceous sediments have been substantially eroded (Fig.
2;Racero-Baena & Drake, 1996; Herngreen & Wong,
2007;Jeremiah et al., 2010). Uplift and erosion were most severe
towardsthe Zandvoort Ridge, where Upper Cretaceous strata have
beensubstantially eroded (Fig. 2). By influencing the present-day
burialdepth and hence temperature of these strata, the inversion
affectedtheir geothermal potential. It also created the horst and
pop-upstructures that were targeted by hydrocarbon wells.
Most of the subsurface data are derived from hydrocarbonfields
on the horst and pop-up structures in the basin. The influenceof
Late Jurassic to Early Cretaceous tectonicmovement on
sedimen-tation as well as the Tertiary inversion complicates
regional well-logcorrelations in the WNB. Therefore, uncertainty
about the geother-mal potential of the Upper Jurassic to Lower
Cretaceous strataremains, despite the wealth of subsurface data
acquired in in the past50 years in the WNB.
WNB stratigraphy, Nieuwerkerk Formation to RijnlandGroup: from
fluvial to marine
The Late Jurassic to Early Cretaceous basin fills in the
Dutchsector of the North Sea basin can be seen as a eustatically
and tec-tonically controlled stepped transgression from the Central
Grabenand Sole Pit Basin in the northwest, towards the Lower
SaxonyBasin and Roer Valley Graben in the southeast (Van
AdrichemBoogaert & Kouwe, 1997; Abbink et al., 2006; Jeremiah
et al.,2010; Munsterman, 2012; Bouroullec et al., 2018;
Verreusselet al., 2018). Essentially, this stepped transgression
was alreadydepicted in the chrono-lithostratigraphic chart of Van
AdrichemBoogaert & Kouwe (1997; Fig. 3), which mimics an
incipientsequence-stratigraphic approach. In every North Sea
sub-basin,
Fig. 1. Geological setting of the West Netherlands Basin,
bordered by the London–Brabant Massif in the south, the Zandvoort
Ridge and Central Netherlands Basin tothe north (after Vondrak et
al. 2018).
2 Cees J.L. Willems et al.
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the following sedimentary facies bands occur either fully or
partly,from bottom to top:
• Terrestrial delta plain sediments, including floodplain fines,
coaland channel sandstones
• Dark lagoonal/marginal claystones, often with
mono-typicalshell assemblage
• Clean stratified barrier sequence sandstones and
transgressionalsheet sands
• Upper shoreface to lower shoreface glauconite-rich
sandstones
Fig. 2. Seismic cross-section of ~40 km, perpendicular to
themain fault trend in theWNB. The interpretation of the faults
(black dotted lines) and top and base of the strata followDuin et
al. (2006).
Fig. 3. Cartoon of regional Upper Jurassic to Lower Cretaceous
stratigraphy on the left, edited after Van Adrichem Boogaert &
Kouwe (1997). On the right, three GR logs tohighlight associated GR
log response.
Netherlands Journal of Geosciences 3
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• Upper offshore to lower offshore glauconite-rich sandstones•
Fully marine silty claystones and occasionally high-TOC (total
organic carbon) claystones
The Upper Jurassic to Lower Cretaceous sediments are groupedinto
lithostratigraphic units. The oldest is the Schieland Group orthe
Niedersachsen Group and predominantly comprises
terrestrialsediments, and the overlying Scruff or Rijnland Group
consists onlyof marine sediments (Fig. 3). Subdivision of Upper
Jurassic to LowerCretaceous sediments follows this approach in all
North Sea basins,but actual nomenclature for groups, formations and
membersdepends on the sub-basin. For example, marine/lagoonal
claystonesfrom the Oyster Ground (Dutch Central Graben),
Neomiodon(Broad Fourteens Basin) and Rodenrijs (WNB) members
arefacies-wise comparable. Similarly, the intensely bioturbated,
glauco-nitic sandstones of the Scruff Greensand Spiculite or
StortemelkFormation in the southern Dutch Central Graben and
TerschellingBasin, as well as the Friesland SstMbr in
theVlielandBasin, are equiv-alents of the Rijn Mbr in the WNB.
Within the WNB, the Upper Jurassic to Lower Cretaceoussequence
is subdivided into the mainly continental SchielandGroup and the
marine Rijnland Group (Fig. 3). Here, theSchieland Group is
represented by the Nieuwerkerk Formation(Van Adrichem Boogaert
& Kouwe, 1997). The NieuwerkerkFormation is traditionally
lithostratigraphically subdivided intothree members based on
sandstone content, abundance of coaland/or frequency of marine
intercalations within differentintervals.
1. The oldest of these members is the Ryazanian AlblasserdamMbr.
It is characterised as a low net-to-gross, syn-rift
fluvialinterval. It unconformably overlies the Upper Jurassic
AltenaGroup in the graben fault blocks and the Lower
JurassicAalburg Formation on some horst blocks (Devault
&Jeremiah, 2002) and has a heterogeneous gamma-ray (GR)log
signature.
2. In the western and central onshore part of the WNB,
theAlblasserdam Mbr is overlain by massive sandstones of
thecoastal-plain Delft Sst Mbr.
3. This member is normally conformably overlain by the
organic-rich claystones of the lagoonal Rodenrijs Claystone
Mbr.
The lithostratigraphic interpretation of a sandstone
packagebelow the Rodenrijs Claystone Mbr belonging to the Delft
orAlblasserdam Mbr without detailed biostratigraphic controlis
exceedingly difficult. From detailed seismic interpretationit
appears that locally a (near-)base Delft Sst Mbr reflectorhas an
angular truncation configuration. Intra-NieuwerkerkFormation
unconformities have been reported (e.g. Devault& Jeremiah,
2002), but not confirmed with accurate seismic-to-well ties.
Directly overlying the Nieuwerkerk Formation are the
marinesediments of the Rijnland Group. Depending on the location in
thebasin, the contact is either conformable or a subtle angular
uncon-formity with an erosional truncation. The sediments were
depos-ited from the Latest Ryazanian in the northwest of the WNB
untilthe Aptian. Its base comprises either claystones of the
VlielandClaystone Formation or sandstones of the Vlieland
SandstoneFormation (Vlieland Sst Fm). Van Adrichem Boogaert &
Kouwe(1997) described several WNB-specific members within
theVlieland Sst Fm, which were deposited as basal transgressive
sands,prograding coastal barrier complexes or offshore shoal
sands
(Fig. 3). Towards the southeast, these marine sandstone
membersgrade into continental claystone deposits, which are often
attrib-uted to the Alblasserdam Mbr. These marine and
terrestrialdeposits are lateral equivalents with rapid lateral
facies variations.Limited well control and geological data create
uncertainty onprediction of the lateral extent, thickness and hence
geothermalpotential of the sandstone-rich members.
Two of the marine sandstone members of the Rijnland Group,the
Rijswijk Mbr and the Berkel Sst Mbr, are currently majorgeothermal
aquifer targets in the WNB (e.g. Vis et al., 2010).The
RijswijkMbrmainly consists of basal transgressive and biotur-bated
sandstones. The Berkel Sst Mbr was deposited by a
regressivecoastal-barrier system prograding to the west and
northwest(Racero-Baena & Drake, 1996). Currently, 11 active
doubletsproduce heat from the Delft Sst Mbr (Mijnlieff, 2020),
making itthe main geothermal target of the WNB.
Because of the syn-tectonic sedimentation, the thickness of
theNieuwerkerk Formation varies significantly in different
faultblocks within the WNB. Accommodation space was created bythe
subsiding grabens and half-grabens. On the basin marginsand horst-
and pop-up structures the thickness ranges from 0 to200 m, while in
some graben fault blocks the NieuwerkerkFormation has a thickness
of up to 1500 m (Duin et al., 2006;Wong, 2007). The Rijnland Group
has a more gradual thicknessdevelopment of c.100 m on the basin
fringes to more than900 m in the centre of the onshore part of the
WNB (Duinet al., 2006; Herngreen & Wong, 2007). This thickness
pattern isconsistent with its setting in a post-rift sequence.
A first sequence-stratigraphic update of this
lithostratigraphicdivision of the WNB by Van Adrichem Boogaert
& Kouwe(1997) was made by Den Hartog Jager (1996). He placed
the strati-graphic members in a sequence-stratigraphic framework.
Thisrefinement was based on interpretation of seismic facies
variationand correlation of net-to-gross trends on well logs.
Later, Devault &Jeremiah (2002) continued with
sequence-stratigraphic updatesof the WNB stratigraphy utilising
palynological analyses of drillcuttings and cores, assisted by
seismic volumes. Devault &Jeremiah (2002) introduced a regional
well-log correlationframework based on maximum flooding surfaces
(MFS) suchas the Forbesi MFS, the Elegans MFS and the Paratollia
MFS.Jeremiah et al. (2010) published a continuation of this work
inwhich the southern North Sea basins were linked using theMFS
approach. More recently Munsterman (2012, 2013) andWillems et al.
(2017c) used new well-log data and palynologicalcuttings analyses
from geothermal wells in the graben fault blocksto improve
understanding of the regional architecture of theDelft Sst Mbr.
Their palynological drill cuttings analyses enabledcorrelation of
chronostratigraphic intervals. This revealed dis-tinctive intervals
of different age within the Delft Sst Mbr as wellas lateral
variation in sandstone content within these intervals,highlighting
that well-log correlation based on the lithostrati-graphic model in
Figure 3, which is based on Van AdrichemBoogaert & Kouwe
(1997), could overestimate the lateral con-tinuity of
sandstone-rich zones.
Aquifer geology: Nieuwerkerk Formation
Depositional environment
Palynological and lithofacies analysis indicates that the
silici-clastic Delft Sst Mbr succession formed in a relatively
humid,lower-coastal-plain meandering-river depositional
environment
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(Donselaar et al., 2015; Vondrak et al., 2018). Variability in
net-to-gross and sandstone stacking patterns in the
NieuwerkerkFormation is the result of relative base-level
fluctuations by differ-ential movement along extensional faults in
combination with ris-ing sea level. Three different net-to-gross
units can be recognised:
1) High net-to-gross units with multi-storey vertical
stackingpattern of meandering-river sandstone bodies reflect
lowaccommodation increase and inherent basinward shift of flu-vial
facies (Fig. 4A). The sedimentary architecture of such
adepositional setting is a sand-prone unit with high verticaland
lateral continuity caused by erosional truncation of theunderlying
floodplain mudstone and fine-grained upper partsof the underlying
sandstone. These sand-prone units havec.40–60% net-to-gross and are
characterised by stackedmulti-storey sandstone bodies with minor
mudstone intervals(Vondrak et al., 2018).
2) Low net-to-gross intervals were formed during a high rate
ofaccommodation increase, which favoured the preservationof
floodplain fines, and consist of interbedded claystone,siltstone
and coal layers with minor thin sandstone inter-calations. In a
sedimentary architecture sense, this impliesa low lateral and
vertical connectivity of the isolatedindividual sandstone units and
preservation of coal layers(Fig. 4B).
3) Units with c.30% net-to-gross consist of loosely
stackedsingle-storey sandstone bodies, characterised by a
sharp,erosional base and fining-upward to claystone and
siltstone.On the GR logs, this is reflected by repeated
fining-upwardcycles (Fig. 4C).
The geothermal aquifers of the Nieuwerkerk Formation consist
ofstacked high and medium net-to-gross units. Their
combinedthickness varies across the basin from ~100 m in the
HON-GT-01well to more than 200 m in the PNA-GT-02 well. Currently,
these
sandstone-prone intervals with locally sandstone content of
morethan 60% are often referred to as the Delft Sst Mbr.
FollowingVan Adrichem Boogaert & Kouwe (1997), this member is
ofValanginian age. Recent palynological drill cuttings
analysesrevealed, however, that such high net-to-gross units do not
neces-sarily extend on a regional scale. They are sometimes
formedby stacking of several high net-to-gross units of different
age, as isshown in Figure 5. This figure shows how a high
net-to-gross intervalin the well PNA-GT-02 is partially Valanginian
and partiallyEarly Valanginian to Late Ryazanian, which is the age
of theAlblasserdam Mbr according to Van Adrichem Boogaert
&Kouwe (1997), and not theDelft SstMbr. This shows that
applicationof the simplified lithostratigraphic model, based on
interpretationof high net-to-gross units alone as sketched in
Figure 3, is notstraightforward. Willems et al. (2017c) explained
the diachronousdevelopment as a result of a shift in fluvial
depocentre from westto east between the Late Ryazanian and Early
Valanginian. In theValanginian phase, a high net-to-gross unit was
formed in thewestern part of the basin, while a low net-to-gross
unit was formedin the eastern part. Conversely, in the Late
Ryazanian / EarlyValanginian,mainly floodplain fineswere deposited
towards thewestand higher net-to-gross intervals were deposited in
the western partof the basin. RecentWNBcorrelation studies show
that palynologicalanalyses are a key tool for improved
understanding of the regionalsandstone distribution within the
Nieuwerkerk Formation and pre-dict aquifer thickness, which is of
paramount importance for doubletdesign. Because such detailed
palynological analyses of NieuwerkerkFormation cuttings from graben
fault blocks are only available for avery limited number of wells,
the traditional lithostratigraphic modelis still dominant in Dutch
geothermal development.
Continued relative sea-level rise in the Hauterivian caused
alandward shift of fluvial facies and preservation of
extensivefine-grained floodplain and swamp sediments of the
RodenrijsClaystone Mbr, finally resulting in lagoonal black
claystone facies.Locally, towards the top of the Rodenrijs
ClaystoneMbr, sandstonebeds are intercalated in a thickening-upward
trend culminating ina sandstone package several metres thick. These
individual sand-stonesmay be represent washover sands in a lagoonal
area behind abarrier (e.g. Vis et al., 2012).
Very few wells intersect complete sections of the
deeperAlblasserdam Mbr in the graben fault blocks. Most
geothermalwells only reach several tens of metres into the top of
this member.The well logs of these wells show loose stacking of
meandering-river sandstone bodies in low net-to-gross intervals,
suggestingsingle-storey fluvial sandstone bodies with low
connectivity, andhence low geothermal potential. Some studies
mention red-beds and braided deposits at the base of the
AlblasserdamMbr (e.g. Den Hartog Jager, 1996) which were, for
example,encountered in the GAAG-06 well (TNO, 2018).
Aquifer sedimentology
The Delft Sst Mbr overlies the Alblasserdam Mbr
conformably,possibly locally disconformably as evidenced by seismic
data.Core evaluations of the Moerkapelle-11 well show that the
basalcontact of the Delft Sst Mbr in that well is marked by a
sharp,erosional surface (Fig. 6A). The overlying Delft Sst Mbr
consistsof a lithofacies association of fine to coarse-gravelly,
moderatelyto poorly sorted, light-grey massive sandstone
interbedded withclaystone, siltstone and coal layers. Individual
sandstone bedsare characterised by a fining-upward grain-size
succession consist-ing of a lag deposit with clay and lignite
clasts in a medium- to
Fig. 4. Subdivision of net-to-gross units within Nieuwerkerk
Formation signaturegamma-ray log (well HON-G-01), with associated
facies architecture. Modified fromDonselaar et al. (2015). Black
curved arrows highlight fining-upward sequences inthe gamma-ray
log.
Netherlands Journal of Geosciences 5
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coarse-sand matrix at the base, to laminated siltstone and coal
atthe top (Fig. 6D). Another core of well Q13-09 contains a
rareexample of a complete fining-upward sequence of c.4 m,
whichcould be related to palaeo bank-full flow depth. Willems et
al.(2017a) used this to estimate the associated bank-full
palaeo-flowwidth of 40 m and meander belt width. FollowingWilliams
(1986),they proposed a 40 m bank-full flow width, and a meander
beltwidth of c.1–2 km.
Rock properties: permeability and porosity
Core plug measurements in the 1500–2500 m depth range show
alinear compaction-related porosity reduction trend of c.5% per500
m (Fig. 7A). The wide spread of porosity values atpresent-day depth
results from facies differences and a varyingdegree of diagenesis
(Fig. 7B). The impact of burial historybecomes evident when
comparing Figure 7A with Figure 7B,which shows the relation between
the maximum burial depthof the core samples and porosity. Maximum
burial depth wasderived from burial maps of the basin-modelling
study byNelskamp & Verweij (2012). Figure 8 shows the relation
between
porosity and permeability for all Nieuwerkerk Formationcore
plugs (kN, Equation 1) and the presumed Delft Sst Mbr(kD, Equation
2):
log10 kNð Þ ¼ �2:03 � 10�7 � ’5 þ 2:547 � 10�5 � ’4 þ 1:035 �
10�3� ’3 þ 8:905 � 10�3 � ’2 þ 0:358 � ’þ 3:21
(1)
log10 kDð Þ ¼ �3:523 � 10�7 � ’5 þ 4:278 � 10�5 � ’4 � 1:723�
10�3 � ’3 þ 1:896 � 10�2 � ’2 þ 0:333 � ’� 3:222
(2)
Considering an average current depth of the sandstones
of2000–2500 m, these plots suggest a porosity of the Delft Sst
Mbrof some 8–25% (Fig. 7B) and associated permeability range
fromseveral tens ofmDup to 3000mD.Measurements in Figures 7 and
8are derived from hydrocarbon wells. So far, well tests from
severalactive geothermal doublets mainly indicate permeability
values ofentire production intervals of over 1000 mD, which is the
higherend of this permeability range. Several hypotheses exist that
explain
Fig. 5. Well section showing thegamma-ray logs of three
geothermalwells in the WNB. High net-to-grossunits forming the
Delft Sst Mbr aquiferare highlighted in each well, togetherwith the
overlying low net-to-grossRodenrijs Claystone Mbr. The Elegansand
Paratollia MFS markers andthe age indications are derived
fromWillems et al. (2017b).
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the high permeability values that were derived from the well
tests ingeothermal wells. Firstly, these doublets exploit younger
intervals ofthe Nieuwerkerk Formation, while the core plug dataset
also con-tains samples from the entire formation. Possibly, the
younger,shallower intervals have more favourable aquifer
properties.Secondly, well tests could show higher permeability
because the
sandstones with highest permeability of the presumed Delft
SstMbr are probably very friable and therefore less frequently
pre-served in the coring process. Finally, the higher well-test
permeabil-ities might be a result of the presence of additional
secondaryporosity from fractures or sub-seismic faults that locally
enhancepermeability. Due to the low vertical resolution of most
log
Fig. 6. Core photographs of the Delft Sst Mbr in well MKP-11.
(A) Erosional contact (dashed line) between grey floodplain
siltstone (top Alblasserdam Mbr) and medium-grainedfluvial channel
sandstonewith coal fragments (base of Delft Sst Mbr). (B)
Oil-stained coarse-grained fluvial sandstonewith lightermud
invasion rim. (C) Oil-stained fine tomedium-grained fluvial
sandstone. (D) Siltstone showing inclined lamination and
interbedded coal; top of the fining-upward succession of a fluvial
sandstone body.
Fig. 7. (A) Porosity – present-daydepth and (B) porosity –
maximumburial depth relations for NieuwerkerkFormation.
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measurements and the low resolution of seismic data,
secondarypermeability is not yet proven. Formation Imaging
loggingand detailed geomechanical modelling could be applied to
elucidatethe possible role of secondary porosity in Nieuwerkerk
Formationaquifers.
Well-test and GR logs also revealed variations in aquifer
qualityand thickness of the Nieuwerk Formation across the basin.The
most favourable flow performance was encountered in thedoublets of
Honselersdijk, Poeldijk and De Lier (Mijnlieff,2020). Aquifer
thickness, as well as aquifer quality, decreasestowards the south
and to a lesser extent also to the north of thesedoublets. As the
clay content and the degree of diagenesis of theDelft Sst Mbr is
typically quite low, a more likely explanationfor this varying
quality could be related to other factors, such asgrain-size
distribution and sorting.
Rock properties: thermal
Mottaghy et al. (2011) published heat conductivity and
densitymeasurements of sandstone and claystone cuttings of
theNieuwerkerk Formation from well KDZ-02 and Q16-02. Theyfound an
average density of 2.68 g cm−3. Heat conductivity mea-surements
were subdivided into samples with heat conductivitybelow 3.0 W m−1
K−1, which are mainly claystone, and sampleswith heat conductivity
higher than 3.0 W m−1 K−1, which wererelated to sandstones. From
these data, mean values of heatconductivity of 2.5 W m−1 K−1 for
the claystone samples and4.5 Wm−1 K−1 for the sandstone samples can
be derived. The heatcapacity or heat diffusivity constant has not
been determined inthese measurements. In the absence of
measurements of theseproperties, so far, rough assumptions were
made for heat capacityand heat conductivity in numerical production
simulations ofgeothermal exploitation of the Nieuwerkerk Formation
aquifer.These assumptions of heat capacity range from 730 J kg−1
K−1
for sandstone and 950 J kg−1 K−1 for claystone by Crooijmans
et al. (2016) to 2700 J kg−1 K−1 for both sandstone and
claystoneby Kahrobaei et al. (2019).
Aquifer geology: Rijnland Group
Depositional environment
The Rijnland Group in the WNB has four distinct sandstonemembers
(Rijswijk, Berkel, IJsselmonde and De Lier), which areall shallow
marine sandstones. Depending on the position onthe coastal
bathymetrical profile and its position with referenceto the
fair-weather and storm wave-base the facies differsignificantly.
The most proximal locations comprise low-anglecross-bedded to
massive, slightly burrowed, clean moderately towell-sorted, fine-
to medium-grained upper shoreface sandstones.GR signature is
generally egg-shaped, with frequent high peakscaused by thin
claystone intercalations.
The more distal facies, deposited as lower shoreface to
loweroffshore sediments, are characterised by medium- to
fine-grainedsilty to clayey, glauconitic sandstones. They are
thoroughly biotur-bated to such an extent that no sedimentary
structures are visibleanymore. Only subtle trends in clay content
hint at an originalstratification of sand and clay, most likely
caused by episodic influxof sand in an area below storm wave-base.
In this area a lowsedimentation rate of predominant background
sedimentationof clay and possibly silt gives the fauna enough time
to churnthe sediment. The GR signature of these facies is
barrel-shaped.
The Rijswijk Sst Mbr represents the first fully marine
sequenceafter the deposition of the terrestrial to marginal marine
sedimentsof the Nieuwerkerk Formation. The Rijswijk Sst Mbr is
depositedas a transgressive sheet, stepping into the WNB from the
north(Jeremiah et al., 2010). Den Hartog Jager (1996) and
Racero-Baena & Drake (1996) described and mapped the marine
trans-gressive sheet sands and coastal barrier sand complexes. The
faciesbelt of the RijswijkMbr has themost northerly position and
did not
Fig. 8. Porosity–permeability cross-plot of Nieuwerkerk
Formation core plug measurements.
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overstep the entire WNB. The younger sandstones
progressivelyencroach the southwest flanks of the WNB, resulting
ultimatelyin configurations where the De Lier Sst Mbr overlies
theNieuwerkerk Formation in the southeastern part of the basin(e.g.
well BRT-01, Fig. 3).
Aquifer sedimentology
The GR log of well VDB-GT-01 exhibits a typical signature of
theRodenrijs Claystone Mbr and Rijswijk Sst Mbr (Fig. 9). Directly
ontop of the claystones a blocky sandstone can be recognised
coveredby a thin claystone interval. This grades into a
barrel-shaped GRresponse, which is associated with lower shoreface
to loweroffshore sandstone. Cores from the Q13-09 and Q13-08 wells
pro-vide insight into the sequence-stratigraphic origin of this
typicalRodenrijs Claystone Mbr GR sequence (Fig. 9; Vis et al.
2012).In the core of well Q13-09, a sequence boundary with an
erosivenature lies on the top of the blocky sand followed by a
verythin transgressive conglomeratic lag. This is associated with
amaximum flooding surface at the maximum shale peak onthe GR. Thus,
the blocky sand below is genetically related to theRodenrijs
Claystone Mbr. After the regional flooding event, thisshale peak
was covered with sandy deposits during a relativesea-level high
stand, represented by the barrel-shaped GR interval.
In cores of the offshore Rijn oilfield the sequence boundary
isfound at the base of the blocky sand, which is characterised by
thepresence of a conglomeratic basal interval (TNO, 2018). The
over-lying claystone and bell-shaped sandstone GR response is
similar
to the interval in Q13-09. In this case the blocky sandstone
maybetter be interpreted as an infill of an erosional topographic
lowin the coastline (incised valley) in which the early
transgressivesands were accumulated and preserved.
Lateral continuity of the sandstones of the Rijnland Group
isexpected to be high. Alberts et al. (2003) showed that
transgressivesheet sandstones like the Rijswijk Mbr, or coastal
barrier complexeslike the Berkel Sst Mbr, could have lateral
continuity of several tensof kilometres, depending on the
palaeobathymetry and the balancebetween sediment supply and
relative base-level. The thickness oftransgressive sheet sandstones
like the Rijswijk Mbr typically doesnot exceed 20 m, while coastal
barrier complexes can build-up to sev-eral tens ofmetres in
thickness (Alberts et al., 2003).Despite the poten-tially high
lateral continuity, well-log correlation is not
alwaysstraightforward in the WNB because of the stepped
transgression.Therefore, similar sandstones (on log response)may
belong to a differ-ent sequence and, in the lithostratigraphic
nomenclature, to a differentmember andmay therefore be laterally
disconnected.Multiple studieshave already shown that
biostratigraphic control is essential to unravelthe lateral facies
architecture on sub-regional scale (Munsterman2012; Vis et al.,
2012; Willems et al., 2017c; Vondrak et al., 2018).
Rock properties
Favourable porosity and permeability values have been measuredin
core plug samples of the marine sandstones of the RijnlandGroup
especially for the Rjiswijk-, Berkel- and IJsselmonde SstMbrs (Fig.
10). According to Vis et al. (2010), there is no significant
Fig. 9. GR logs of wells Q13-08 and Q13-09 offshore The Hague,
as well as core section from Q13-09 highlighting that boundary
between the Rijnland Group and NieuwerkerkFormation is not always
marked by a transition from high to low GR readings. The top of the
Nieuwerkerk Formation could contain sandy units, complicating
identification of theboundary based on GR logs alone.
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relationship between depth and permeability for the
RijnlandGroup sands in this basin. This is reflected by the large
scatteron the porosity–permeability cross-plots of Figure 10.
Possibleexplanations include (1) the facies differences within the
differentmembers, (2) varying diagenesis as a result of differences
in burialhistory (Vis et al. 2010) and (3) the limited maximum
burial depthof up to 2500 m. Especially for the Rijswijk Mbr, the
permeabilitydeclines more rapidly for lower porosity values, which
couldbe attributed to the effect of dispersed clay and
glauconite.This reduces its geothermal potential. Despite the many
availableporosity–permeability measurements of sandstone members
ofthe Rijnland Group, the authors of this paper are not aware of
pub-lications on thermal properties of the Rijnland Group
aquifers.
Geological challenges
Even in the data-rich WNB, geothermal power estimates for
newdoublets still have a wide uncertainty range. This is reflected
bythe large spread of P10 to P90 capacity estimates for new
doubletsof several MWth. The uncertainty has a potential negative
impacton thematuration of these projects because the risk puts off
investors,especially as the geological insurance- and feed-in
subsidy allocationsare based on these estimates. The uncertainty in
capacity estimatesresults from a limited understanding of (1) the
regional sedimentaryaquifer architecture, (2) sub-seismic
structural geology and (3) aqui-fer rock properties. A better
understanding of these three geologicalparameters is also crucial
for safe and sustainable exploitation of theresources and is a
prerequisite for optimisedwell planning in the lim-ited space
available between currently active doublets. Themain chal-lenges
and uncertainties are discussed in the following paragraphs.
Uncertainty in sedimentary aquifer architecture
Despite the increasing number of studies with
sequence-stratigraphic updates, the coarse lithostratigraphic model
of VanAdrichem Boogaert & Kouwe (1997) in Figure 3 still forms
the
basis for geothermal doublet planning and design in the WNB.The
development of higher-resolution models based on
sequencestratigraphy has been limited by the limited amount of
geologicaldata acquired from geothermal wells. Firstly, logging in
geothermalwells is less extensive than logging in hydrocarbon
wells, or onlydone for sections of the wellbores. Also, no vertical
seismic profiles(VSPs) are available in geothermal offset wells.
Therefore, nowell-to-seismic ties can be made, which limits the
available infor-mation that could be derived from seismic data on
the regionalsedimentary architecture. New geothermal well planning
wouldbenefit from enhanced used of seismic data, especially
becausereprocessed WNB vintage seismic surveys are now available
inthe public domain that dramatically improved imaging of
sedi-mentary and tectonic features. Secondly, geothermal wells in
theWNB have relatively simple completions compared to their
hydro-carbon counterparts. They have large open-hole sections
withproduction tubing and screens and no downhole flow sensors
toidentify differences in productivity of intervals. Therefore,
uncer-tainty remains as to the net aquifer thickness, which is a
keyelement of sedimentary aquifer architecture. This affects the
pre-diction of lifetime and interference (Mijnlieff & Van Wees,
2009;Hamm & Lopez, 2012; Crooijmans et al., 2016), and
heat-in-placeestimates (Vis et al., 2010; Kramers et al., 2012).
Intervals withhigher flow rates can also be more subject to
erosion, scalingand corrosion, and therefore the simple completions
also restrictwell-integrity studies. For detailed local aquifer
zonation evalu-ation without the availability of downhole flow
sensors, repetitiveproduction logging is a prerequisite to
delineate the net aquiferthickness and allow more detailed
permeability calculations fromwell tests. Finally, no cores have
been derived from the UpperJurassic to Lower Cretaceous interval in
the graben fault blocks.These would not only allow high-resolution
studies of the sedi-mentary aquifer architecture, they would also
permit rock propertymeasurements. For example, geothermal core plug
permeabilitymeasurements could elucidate the difference between
hydrocarboncore plug measurements and the permeability derived
from
Fig. 10. Porosity–permeability cross-plotof Vlieland Sandstone
Members of theRijnland Group: (A) Berkel Sst; (B) RijswijkSst; (c)
De Lier Sst; (D) IJselmonde Sst.
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geothermal well tests. Not all geothermal wells would need to
becored. Palynological analysis of drill cuttings (e.g.
Munsterman2012, 2013; Willems et al., 2017c) and field work
analogue studies(e.g. Flood & Hampson, 2015; Owen et al., 2018)
could assist inextrapolating observations in core studies to a
regional scale.So far, palynological analyses have only been
applied to a verylimited number of geothermal and petroleum wells
in the WNB,and outcrop analogue studies have not been reported in
theliterature.
Uncertainties in structural geology and thermal properties
Uncertainty not only remains for the regional sedimentary
archi-tecture, but also for the existence and distribution of
sub-seismicfaults and fractures in the Upper Jurassic to Lower
Cretaceousinterval. These features could act as flow baffles or
alternativelyas high-permeability short cuts, affecting thermal
breakthroughtime and optimised doublet deployment (e.g. Mijnlieff
&Van Wees, 2009). An improved understanding of the variancein
thermal properties of the rocks is required for more
adequatepredictions of thermal breakthrough as a basis for
optimised doubletdesign. Heat conductivity and heat capacity affect
the cold-waterfront propagation, the cold-water breakthroughmoment
and the rateof the production temperature decline thereafter (e.g.
Poulsen et al.,2015; Pandey et al., 2018). The variability of these
properties couldbe large (e.g. Fuchs, 2018) and has not been
determined for WNBaquifer rocks. A better grip on reservoir
properties, including porosity,permeability, geomechanical and
thermal properties, can be derivedfrom core samples.
Value of information in geothermal operations
The geological uncertainties mentioned above continue to a
largeextent, as a result of cost cutbacks for data acquisition and
geologi-cal studies in geothermal operations. It should be noted
that thepast decade has seen several examples of geothermal wells
thathave had adequate to good data acquisition campaigns
includingextensive logging of Upper Jurassic – Lower Cretaceous
strata.A subtle trend of more data acquisition is emerging
recognisingthe need for data-driven static and dynamic subsurface
geologicalmodels for long-term efficient and safe operations of
geothermalsystems. Nevertheless, hydrocarbon developers typically
acquiremuch more subsurface data than geothermal operators
becauseof the higher value of hydrocarbons compared to the value of
geo-thermal heat. In addition, there was limited incentive for the
firstgeothermal operators in the WNB to invest in data because
theyonly aimed to provide heat for local, decentralised heat
networkswith a single doublet. In contrast, hydrocarbon operators
aim toexploit an entire resource with dozens of wells. Because
dataacquisition from the first wells reduces the risk of failure of
laterwells, there is a much stronger incentive for extensive data
acquis-ition in regionally focused operations. Because of the
differentexploitation standards in both industries, and because of
differentfinancial boundary conditions, we expect that geothermal
opera-tors cannot directly copy data acquisition standards from
hydro-carbon operators. Therefore, a major challenge for
optimisinggeothermal exploitation is to quantify the value of
information(VOI) of data, to be able to derive new geothermal data
acquisitionstandards. In particular, coring and seismic inversion
are too costlyfor individual operators and should be shared. VOI
could be a basisfor long-term data cost-sharing strategies between
neighbouringoperators as well as government agencies.
New aquifer targets: Alblasserdam Member
The Alblasserdam Mbr is generally considered as a low
net-to-gross interval below the Delft Sst Mbr with a low
geothermalpotential. Devault & Jeremiah (2002) and Jeremiah et
al. (2010)did not recognise the Delft Sst Mbr as a separate unit
and believedthat amalgamated sandstone complexes occur throughout
theNieuwerkerk Formation. This suggests not only that the DelftSst
Mbr of Van Adrichem Boogaert & Kouwe (1997) is a
possiblegeothermal target in the formation, but that additional
similaramalgamated sandstone complexes could occur locally in
deeperparts of the Alblasserdam Mbr. The sandstone distributionin
the Alblasserdam Mbr is poorly understood because mostwells only
intersect several hundred metres of the top of theNieuwerkerk
Formation. Den Hartog Jager (1996) described theseismic facies of
the Alblasserdam section as discontinuous, withlow amplitudes. He
associated this with rapid lateral facies varia-tion, abundant
impermeable shale flow baffles and barriers andlower sandstone
content and braided deposits at the base of themember. Because of
the syn-tectonic origin, it is not straightfor-ward to use well
data from this member from hydrocarbon wellson structural highs to
predict the geothermal potential of theAlblasserdamMbr in the
graben blocks. Moreover, these structuralelements are often heavily
faulted, which complicates well-logcorrelation of the sandstone
members. The limited availablecore- and well-test data also suggest
better aquifer quality of theDelft Sst compared to that of the
Alblasserdam Mbr. Mainly thehigher temperatures due to the
increased depth of up to 3000 m(Duin et al., 2006) still make the
Alblasserdam Mbr a considerablepotential geothermal target in the
WNB, while the increased depthmay also influence the permeability
and porosity unfavourably.
New aquifer targets: Rijnland Group
Despite its favourable permeability, porosity and large
volumes(Vis et al., 2010), the Rijnland Group is currently
underdeveloped,with only one doublet producing heat from sandstones
belongingto this group. This could be explained by their generally
shallowerdepth with lower associated temperatures. Current
operators’ geo-thermal exploitation focus is on deeper targets with
temperaturesmore than 70°C. Future interest in the Rijnland Group
mightincrease with growing demand for low-carbon heat and
lowerfuture thresholds for minimal required production
temperature.Progressive heat exchanger efficiency and improved
thermal insu-lation of greenhouses and buildings will further
reduce minimumrequired production. Alternatively, exploitation of
the RijnlandGroup could be enhanced when imminent interference
betweenoperators of the Nieuwerkerk Formation forces new operators
tolook for alternatives. This highlights that the expansion of
geother-mal exploitation in the WNB not only depends on
geologicaluncertainties but also on legislative measures to deal
with them.
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Geology of the Upper Jurassic to Lower Cretaceous geothermal
aquifers in the West Netherlands Basin - an
overviewIntroductionStructural geological setting, West Netherlands
BasinWNB stratigraphy, Nieuwerkerk Formation to Rijnland Group:
from fluvial to marineAquifer geology: Nieuwerkerk
FormationDepositional environmentAquifer sedimentologyRock
properties: permeability and porosityRock properties: thermal
Aquifer geology: Rijnland GroupDepositional environmentAquifer
sedimentologyRock properties
Geological challengesUncertainty in sedimentary aquifer
architectureUncertainties in structural geology and thermal
propertiesValue of information in geothermal operationsNew aquifer
targets: Alblasserdam MemberNew aquifer targets: Rijnland Group
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